NMR observation of mixed complex formation and stereochemical non-rigidity in cobalt(II) complexes

Abstract

Contact shift measurements of aqueous solutions containing a 1:1:1 moi ratio of Co(II): dien:asp (dien = NH 2CH 2CH 2NHCH 2CH 2NH 2, Asp = NH 2CH(CH 2CO 2H)CO 2H) indicate that at pD's above 7.5 the predominant species (>97%) in aqueous solution is Co(dien)(Asp)(D 20). This represents approximately a 4 Kcai mol −1 preference for the heterogenous complex. At lower pH's protonation of the dien remits in lots of dien and production of a significant amount of Co(Asp)(D 20) 3 andCo(Asp) −2 2. The mixed ligand complex is stereochemically non-rigid at 34°C with the asp portion undergoing rapid interconversion between isomers which differ in the carboxylate that is coordinated to the metal. At −14°C, this interconversion is slow on the nmr time scale. At 100°C, a bond rupture process becomes rapid which results in inversion of the dlen nitrogens and stereochemical nonrigidity of the allen side. The complex Co(dien)(D 2) 2+ is found to exist solution as approximately 67% meridonal and 33% facial form which are in rapid equilibrium with each other at 34 °C but slowly interconvert at −5°C. This process is found to involve breakage of a cobalt nitrogen bond.